1. Field of the Invention
The present invention relates generally to signal processing. More particularly, the invention relates to circuits and methods for arbitrating relative timing between two waveforms.
2. State of the Art
Known signal processors include coincidence detector circuits, race arbiter circuits and time difference analyzers having semiconductor latching circuits. For example, these signal processors are used in frequency or phase modulation systems (e.g., FM or PM modulation domain analyzers) to detect a signal's frequency or phase relative to that of a reference signal.
A typical coincidence detector circuit is a latching AND gate which detects the outputs from two separately triggered pulse generators. A typical race arbiter circuit is an arrangement of flip-flops that identifies which of two signals is received first. A time difference analyzer can be formed as a combination of semiconductor logic gates and a digital timer (e.g., counter) which detects the phase difference between a received signal and a reference signal.
Although simplistic in architecture, the foregoing circuits suffer significant drawbacks. For example, semiconductor latching circuits (e.g., latching AND gates) are limited to the detection of relatively slow (i.e., low frequency) input signals with relatively long pulse widths. The resolution of these circuits becomes increasingly limited as higher frequency signals of shorter pulse width are received. Similarly, flip-flops are typically limited to use with relatively slow signals, and can be especially unstable under certain circumstances (e.g., noisy environments).
To address the need for faster, more accurate signal processing, coincidence detector circuits have been proposed which include multipliers for multiplying two input signals together. The resulting signal is processed with a low pass filter (i.e., integrator) and threshold detector. While such circuits have achieved improved time resolution (e.g., 20 ps resolution), the threshold of the threshold detector must be readjusted any time the input signal waveforms are changed. These circuits therefore require constant monitoring and readjustment and are limited to the resolution mentioned above. Further, these circuits are limited to detecting sharply pulsed input signals due to the inherent background noise generated by the multiplier.
Other areas of signal processing have more recently focused on the use of superconducting electronics to improve speed and resolution. For example, known superconducting devices are capable of highly accurate magnetic flux detection. However, these devices have been limited to relatively basic signal detection. Superconducting devices have not, for example, been employed in more sophisticated signal processing such as arbitrating the relative timing between two waveforms.
Two known superconducting devices are the Superconducting Quantum Interference Device (SQUID) and the Quantum Flux Parametron (QFP). Both of these devices are signal comparators capable of detecting magnetic flux with high resolution.
Both the SQUID and the QFP achieve high resolution flux detection through the use of Josephson-junction circuit elements (i.e., Josephson junctions). Josephson junctions are described in a document entitled "Superconducting electronics", Physics Today, Feb. 1981 by Donald G. McDonald, the disclosure of which is hereby incorporated by reference in its entirety.
As described in the aforementioned document, superconducting loops which include Josephson junctions are devices which exploit the concept of magnetic flux quantization. Generally speaking, magnetic flux quantization refers to the ability of superconducting loops, or rings, to trap the magnetic field of a permanently circulating supercurrent in discrete units.
Josephson junctions are typically formed from two thin films of superconducting metals separated by a thin insulating layer. An electrical current is conducted across the two thin films. A zero voltage drop occurs across the films when current is below a predetermined maximum level referred to as the "critical current". Currents which exceed the critical current (e.g., approximately 1 mA) produce a voltage drop across the two thin films.
A SQUID is a superconducting loop which is interrupted by two Josephson junctions. Maximum current through the superconducting loop occurs when either junction reaches its critical current. Because the maximum current is a periodic function of the magnetic flux through the SQUID, these devices provide a finely graded measuring scale for magnetic flux detection. The frequency with which the maximum current is detected (i.e., the frequency with which a voltage drop is detected across at least one Josephson junction) represents a measure of magnetic flux through the SQUID.
A document entitled "A Single-Chip SQUID Magnetometer", IEEE Transactions On Electron Devices, Vol. 35, No. 12, Dec. 1988 by Norio Fujimaki et al. further describes a SQUID magnetometer. The SQUID magnetometer includes a sensor which changes from a zero-voltage state to a finite voltage state (e.g., approximately 1 mV) when an AC bias current pulse crosses a threshold value. The threshold value is a function of the magnetic flux coupled to the SQUID and depends upon characteristics of the SQUID sensor (i.e., the inductance, the Josephson junction critical currents and the location of the bias current injection point).
U.S. Pat, No. 4,916,335 (Goto et al), the disclosure of which is hereby incorporated by reference in its entirety, discloses a QFP for polarity discrimination of an input signal. As described therein, a QFP is a parametron-type switching circuit which includes a superconducting loop interrupted by two Josephson junctions positioned on opposite sides of a center node. Like the SQUID, the QFP is a highly accurate magnetic flux detector capable of amplifying weak magnetic flux. As the center node inductance of a QFP is increased, its functional behavior approaches that of a SQUID (i.e., the two Josephson junctions become decoupled from one another).
Although superconducting devices such as SQUIDs and QFPs offer the advantages of high speed and high resolution flux detection, these advantages have not been effectively exploited beyond mere signal detection and polarity discrimination. As described above, signal processors are often required to detect and measure the time delays between plural signal waveforms. Despite the availability of superconducting devices, signal processors presently employ conventional arbiter circuits, coincidence detectors and time difference analyzers. These signal processors are therefore limited in their overall speed of operation and flexibility due to restrictions on the input signal waveforms.
Accordingly, there is a need for a signal processor capable of arbitrating a wide range of input signals with high resolution.